Amazing Spider-Man 3 - This would be the Black Costume story. Eddie Brock could be introduced as a reporter for the Daily Bugle, as a way to bring Peter Parker to the Bugle. Because first you'd have to introduce Brock, develop his character a bit, explain why Brock comes to hate Spider-Man. You'd also have to have Peter wear the symbiote, you have to show why he chooses to get rid of it and not keep it. Then you have the symbiote merge with Brock, the two of them become Venom, leading to an appearance in #4. This could explain the relationship between Peter and the symbiote, making the transition into a fourth title starring Venom easier.

Amazing Spider-Man 4 - Now we have an entire movie of Spidey versus Venom, with Venom having been established in the franchise with a proper background. Movie ends with Brock and the symbiote being separated and Brock getting locked up. Carnage is saved for the spinoff.

That's how I'd do it. This was actually fun imagining, and I think I'll write these out as fan fics. What do you people think?